A group of diseases caused by resistance to PTH (parathormone) leading to hypocalemia & hyperphosphatemia accompanied by increased PTH levels in serum (negative feedback).
Patients with type Ia have a genetic deficiency of the Gs protein that couple PTH receptors to adenyl cyclase. These patients have in addition to the manifestations of hypocalcemia and hyperphosphatemia a peculiar constellation of somatic characteristics including subcutaneous ossifications, brachydactyly (short metacarpals), obesity, round facies, and short stature.
In some subjects, hypocalcemia and hyperphosphatemia are associated with radiographically evident osteitis fibrosa cystica (usually a finding of hyperparathyroidism). This finding suggests selective renal, as opposed to skeletal, resistance to PTH action. The pathogenesis is unknown.
Clinical suspicion
Hypocalcemia and hyperphosphatemia accompanied by increased PTH levels and normal renal function.
Diagnosis
PTH infusion (at present with commercially available synthetic 1-34 peptide) and measurement of urinary cAMP excretion.
Physical appearance can help distinguish type Ia from type Ib pseudohypoparathyroidism.
